Piezoelectric composition and piezoelectric device

ABSTRACT

wherein 0.5≤x≤0.8, 0.02≤y≤0.4, 0.02≤z≤0.2, x+y+z=1, and 0.96≤m≤1.04.

TECHNICAL FIELD

The present invention relates to a piezoelectric composition and apiezoelectric device.

BACKGROUND

A perovskite-type metal oxide is known as a common piezoelectriccomposition. The constitution of a perovskite-type metal oxide isrepresented by ABO₃. A perovskite-type piezoelectric composition is, forexample, lead zirconate titanate (Pb(Zr, Ti)O₃). The curie temperature(T_(c)) of lead zirconate titanate (PZT) is high, and the piezoelectricconstant (d₃₃) of PZT is large. However, PZT does harm to theenvironment or the human body due to containing lead as an element at Asites. The piezoelectric composition not containing lead is required inview of influence on the environment or the human body.

An example of the piezoelectric composition not containing lead isbismuth ferrite (BiFeO₃) described in the following Non PatentLiterature 1. The Tc of bismuth ferrite (BFO) is high, and BFO exhibitslarge spontaneous polarization. However, in the case of BFO alone,enough piezoelectric performance (for example, d₃₃) is not obtained dueto the anisotropy being high, the leakage current being large and thelike.

Therefore, a piezoelectric composition the Tc of which is high and thed₃₃ of which is large is required. A binary compound composed of bariumtitanate and bismuth ferrite is disclosed in the following Non PatentLiterature 2. A ternary compound composed of barium titanate, bismuthferrite and a composite oxide such as bismuth magnesate titanate isdisclosed in Japanese Unexamined Patent Publication 2013-191751.

-   [Non Patent literature 1] Tadej Rojac et al., “Strong ferroelectric    domain-wall pinning in BiFeO3 ceramics”, JOURNAL OF APPLIED PHYSICS,    108, 074107, 2010.-   [Non Patent literature 2] Zhenyong Cen et al., “Effect of sintering    temperature on microstructure and piezoelectric properties of    Pb-free BiFeO3-BaTiO3 ceramics in the composition range of large    BiFeO3 concentrations”, J Electroceram, 31, p. 15-20, 2013.

SUMMARY Problem to be Solved by the Invention

A piezoelectric composition described in Japanese Unexamined PatentPublication 2013-191751 is greatly distorted when an electric field highenough is impressed thereon. However, it is difficult for apiezoelectric composition described in Japanese Unexamined PatentPublication 2013-191751 to have a piezoelectric constant large enoughafter a poling process is performed thereon. Furthermore, when a polingprocess is performed on a piezoelectric composition described in NonPatent Literature 2, the d₃₃ of the piezoelectric composition after thepoling process is around 130 pC/N, and is small.

Therefore, the present invention has been completed in view of the abovecircumstances, and an object of the present invention is to provide apiezoelectric composition the d₃₃ of which is large and the specificresistance of which is large and a piezoelectric device using thepiezoelectric composition.

Means for Solving the Problem

A piezoelectric composition according to one aspect of the presentinvention is represented by the following Chemical Formula (1):

x[Bi_(m)FeO₃]-y[Ba_(m)TiO₃]-z[Sr_(m)TiO₃]  (1)

wherein 0.5≤x≤0.8, 0.02≤y≤0.4, 0.02≤z≤0.2, x+y+z=1, and 0.96≤m≤1.04.

A piezoelectric device according to one aspect of the present inventioncomprises the above piezoelectric composition.

Effects of Invention

According to the present invention, a piezoelectric composition the d₃₃of which is large and the specific resistance of which is large and apiezoelectric device using the piezoelectric composition are provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a piezoelectric device according to oneembodiment of the present invention.

FIG. 2 is a schematic view illustrating the constitutions ofpiezoelectric compositions in Examples and Comparative Examples.

DETAILED DESCRIPTION

Preferred embodiments of the present invention will be described indetail hereinafter with proper reference to the drawings. In thedrawings, the identical or equivalent components are indicated with theidentical reference sign. When a description overlaps, the descriptionis omitted. The present invention is not limited to the followingembodiments.

As shown in FIG. 1, a piezoelectric device 10 according to thisembodiment comprises a pair of electrodes 4 a and 4 b and apiezoelectric body 2 between the pair of electrodes 4 a and 4 b. Thatis, an electrode 4 a overlaps one surface of the piezoelectric body 2,and a different electrode 4 b overlaps the other surface of thepiezoelectric body 2.

A piezoelectric composition contained in the piezoelectric body 2 isrepresented by the following Chemical Formula (1). The piezoelectricbody 2 may consist of only the piezoelectric composition represented bythe following Chemical Formula (1). The piezoelectric body 2 may containother ingredients in addition to the piezoelectric compositionrepresented by the following Chemical Formula (1). The piezoelectriccomposition represented by the following Chemical Formula (1) may be apowder, or may be a sintered body.

x[Bi_(m)FeO₃]-y[Ba_(m)TiO₃]-z[Sr_(m)TiO₃]  (1)

[In Formula (1), 0.5≤x≤0.8, 0.02≤y≤0.4, 0.02≤z≤0.2, x+y+z=1, and0.96≤m≤1.04].

The piezoelectric composition represented by the above Chemical Formula(1) may have a perovskite-type crystal structure. The piezoelectriccomposition represented by the above Chemical Formula (1) may be acomposite oxide represented by the following Chemical Formula (2). Thepiezoelectric body 2 may consist of only the piezoelectric compositionrepresented by the following Chemical Formula (2). The piezoelectricbody 2 may contain other ingredients in addition to the piezoelectriccomposition represented by the following Chemical Formula (2).

(Bi_(x)Ba_(y)Sr_(z))_(m)(Fe_(x)Ti_((y+z)))O₃  (2)

[In Formula (2), 0.5≤x≤0.8, 0.02≤y≤0.4, 0.02≤z≤0.2, x+y+z=1, and0.96≤m≤1.04].

A portion of the piezoelectric composition may be a phase consisting ofBi_(m)FeO₃. A portion of the piezoelectric composition may be a phaseconsisting of Ba_(m)TiO₃. A portion of the piezoelectric composition maybe a phase consisting of Sr_(m)TiO₃. A portion of the piezoelectriccomposition may be a solid solution of at least two oxides selected fromthe group consisting of Bi_(m)FeO₃, Ba_(m)TiO₃ and Sr_(m)TiO₃.

The present inventors consider that a reason the d₃₃ and the specificresistance of the above piezoelectric composition are large is asfollows. In a process of sintering perovskite-type BiFeO₃ to form thepiezoelectric body 2, Bi is likely to volatilize. Also in a process offorming a thin film containing BiFeO₃, Bi is likely to volatilize. Forexample, when a thin film is formed by a sputtering method, Bi isunlikely to be incorporated into the thin film in a growth process.Therefore, in conventional BiFeO₃, which does not contain Sr, Bi islikely to be deficient and vacancies are likely to occur at A sites ofBiFeO₃. That is, in conventional BiFeO₃, defects at A sites are likelyto occur. Meanwhile, a piezoelectric composition according to thisembodiment contains Sr, which is unlikely to volatilize. Therefore,vacancies at A sites caused by the volatilization of Bi are replacedwith Sr, and the defects at A sites are unlikely to occur. Consequently,the specific resistance of the piezoelectric composition according tothis embodiment is larger than the specific resistance of conventionalBiFeO₃. Furthermore, the piezoelectric composition according to thisembodiment is formed from rhombohedral Bi_(m)FeO₃, tetragonal Ba_(m)TiO₃and cubic Sr_(m)TiO₃. Therefore, polarization rotation becomes likely tooccur near the boundaries of the three phases of Bi_(m)FeO₃, Ba_(m)TiO₃and Sr_(m)TiO₃. Consequently, the d₃₃ of the piezoelectric compositionaccording to this embodiment is larger than the d₃₃ of conventionalBiFeO₃. In addition, the reason the d₃₃ and the specific resistance ofthe piezoelectric composition according to this embodiment are large isnot limited to the above reason.

In the above Chemical Formula (1), the x satisfies 0.5≤x≤0.8. When the xis less than 0.5, the crystal structure of the piezoelectric compositionshifts to a cubic crystal, and the d₃₃ of the piezoelectric compositionis likely to become small. When the x is more than 0.8, the crystalstructure of the piezoelectric composition shifts to a rhombohedralcrystal, polarization reversal is unlikely to occur, the d₃₃ of thepiezoelectric composition becomes small, many defects at A sites arecaused by the volatilization of Bi, and the specific resistance islikely to decrease. It is preferable that the x satisfy 0.55≤x≤0.75.Since the x is in the above numerical value range, the d₃₃ and thespecific resistance of the piezoelectric composition are likely tobecome large.

In the above Chemical Formula (1), y satisfies 0.02≤y≤0.4. When the y isless than 0.02, the crystal structure of a piezoelectric compositionshifts to a rhombohedral crystal, the polarization reversal is unlikelyto occur, and the d₃₃ of a piezoelectric composition becomes small, manydefects at A sites are caused by the volatilization of Bi, and thespecific resistance is likely to decrease. When the y is more than 0.4,the crystal structure of the piezoelectric composition shifts to a cubiccrystal, and the d₃₃ of the piezoelectric composition is likely tobecome small. It is preferable that the y satisfy 0.15≤y≤0.33. Since they is in the above numerical value range, the d₃₃ and the specificresistance of the piezoelectric composition are likely to become large.

In the above Chemical Formula (1), z satisfies 0.02≤z≤0.2. When the z isless than 0.02, the defects at A sites caused by the volatilization ofBi are not fully replaced with Sr. Consequently, the specific resistanceis likely to decrease. When the z is more than 0.2, the crystalstructure of the piezoelectric composition shifts to a cubic crystal,and the d₃₃ of the piezoelectric composition is likely to become small.It is preferable that the z satisfy 0.02≤z≤0.1. Since the z is in theabove numerical value range, the d₃₃ and the specific resistance of thepiezoelectric composition are likely to become large.

In the above Chemical Formula (1), the m satisfies 0.96≤m≤1.04. When them is less than 0.96, Bi, Ba and Sr, which are at A sites, decrease,defects at A sites increase, and the specific resistance is likely todecrease. When the m is more than 1.04, heterogeneous phases containingany of Bi, Ba and Sr are formed in grain boundaries, and the d₃₃ of thepiezoelectric composition is likely to become small. It is preferablethat the m satisfy 0.98≤m≤1.02. Since the m is in the above numericalvalue range, the d₃₃ and the specific resistance of the piezoelectriccomposition are likely to become large.

The above Chemical Formula (2) is represented as A_(m)BO₃. The m is theratio [A]/[B] of the number [A] of all atoms occupying A sites to thenumber [B] of all atoms occupying B sites. When the m is out of theabove numerical value range, the m, namely the [A]/[B], greatly deviatesfrom a stoichiometric ratio of 1, and therefore defects in thepiezoelectric composition are likely to increase and heterogeneousphases are likely to increase in grain boundaries, thereby the d₃₃ andthe specific resistance of the piezoelectric composition are likely tobecome small.

The piezoelectric composition may contain other elements than theelements contained in the above Chemical Formula (1) as impurities orminute amounts of additives in the form of compounds or simplesubstances. Examples of such a compound include oxides of Na, Al, Si, P,K, Fe, Cu, Zn, Hf, Ta, or W. When the piezoelectric composition containsthese oxides and the like, it is preferable that the total content ofoxides in the piezoelectric composition be 0.3% by mass or less of thewhole piezoelectric composition in terms of oxides of the elements. Thatis, it is preferable that the main ingredient, specifically 99.7% bymass or more of the whole piezoelectric composition, have theconstitution represented by Chemical Formula (1). In this case, thepiezoelectric composition substantially has the constitution representedby Chemical Formula (1).

The constitution of the piezoelectric composition can be measured, forexample, by an X-ray fluorescence analysis (XRF method).

Since the piezoelectric device 10 comprises the piezoelectric body 2containing the piezoelectric composition represented by the aboveChemical Formula (1), it is excellent in piezoelectric characteristics.The piezoelectric body 2 may be a sintered body containing the abovepiezoelectric composition, or may be a thin film containing the abovepiezoelectric composition.

The potential difference between a pair of electrodes 4 a and 4 b maybe, for example, 0.1 to 2.0 kV/mm. In a piezoelectric device usingconventional BiFeO₃, as long as the potential difference between theelectrodes of the piezoelectric device is not a high voltage of 5.0kV/mm or more, enough piezoelectric characteristics are not obtained.Meanwhile, in the piezoelectric device 10 according to this embodiment,even if the potential difference between the electrodes 4 a and 4 b is alow voltage in the above numerical value range or a high voltage higherthan the above numerical value range, enough piezoelectriccharacteristics can be obtained.

Then, an example of a method for manufacturing the piezoelectric device10 according to this embodiment will be described. The method formanufacturing the piezoelectric device 10 comprises: a mixing step ofgranulating a raw material powder of the piezoelectric body 2 containingthe above piezoelectric composition; a sintering step of press-formingthis raw material powder to form a formed body and sintering the formedbody to produce a sintered body; and a polarization step of performing apoling process on the sintered body to obtain the piezoelectric device10. Steps will be described specifically hereinafter.

In a mixing step, starting raw materials for preparing the piezoelectriccomposition are first provided. As the starting raw materials, oxides ofelements that constitute the piezoelectric composition represented bythe above Chemical Formula (1) or compounds that become these oxidesafter sintering (carbonates, hydroxides, oxalates, nitrates and thelike) can be used. As specific starting raw materials, a Bi (bismuth)compound, a Fe (iron) compound, a Ba (barium) compound, a Ti (titanium)compound, a Sr (strontium) compound and the like may be used. Thesestarting raw materials are blended at a molar ratio or a mass ratio suchthat the piezoelectric composition represented by the above ChemicalFormula (1) is formed after sintering, and wet mixing is conducted by aball mill or the like.

The Bi compound may be bismuth oxide (Bi₂O₃), bismuth nitrate (Bi(NO₃)₃)or the like. The Fe compound may be iron oxide (Fe₂O₃), iron chloride(FeCl₃), iron nitrate (Fe(NO₃)₃) or the like. The Ba compound may bebarium oxide (BaO), barium carbonate (BaCO₃), barium oxalate (C₂BaO₄),barium acetate ((CH₃COO)₂Ba), barium nitrate (Ba(NO₃)₂), barium sulfate(BaSO₄), barium titanate (BaTiO₃) or the like. The Ti compound may betitanium oxide (TiO₂) or the like. The Sr compound may be strontiumcarbonate (SrCO₃) or the like.

Then, the mixed raw material obtained by wet mixing is temporarilyformed into a temporarily formed body, which is then calcined. Acalcined body containing the above piezoelectric composition is obtainedby this calcination. It is preferable that the calcination temperaturebe 600 to 900° C., and it is preferable that the calcination time bearound 1 to 16 hours. When the calcination temperature is too low, achemical reaction tends not to proceed fully in the temporarily formedbody. When the calcination temperature is too high, a temporarily formedbody begins to sinter, and subsequent pulverization therefore tends tobecome difficult. Calcination may be performed in the air atmosphere,and may be performed in an atmosphere in which the partial pressure ofoxygen is higher than that in the air, or a pure oxygen atmosphere.Furthermore, the wet-mixed starting raw material may be calcined withoutbeing temporarily formed.

Then, the obtained calcined body is slurried and pulverized (wet ground)by a ball mill or the like, and thereafter a fine powder is obtained bydrying the slurry. A binder is added to the obtained fine powder ifneeded, and the raw material powder is granulated. It is preferable touse water, an alcohol such as ethanol, a mixed solvent of water andethanol, or the like as a solvent for slurrying a calcined body. Bindersadded to the fine powder include commonly used organic binders such aspolyvinyl alcohol, polyvinyl alcohol to which a dispersing agent isadded, ethyl cellulose, and the like.

In a sintering step, a formed body is formed by press-forming thegranulated raw material powder. A load at the time of press-forming maybe adjusted, for example, to 1.0 to 3.5 MPa.

Then, binder removal treatment is provided to the obtained formed body.It is preferable that the binder removal treatment be performed at atemperature of 400 to 800° C. for around 2 to 4 hours. The binderremoval treatment may be performed in the air atmosphere, or may beperformed in an atmosphere in which the partial pressure of oxygen ishigher than that in the air, or a pure oxygen atmosphere.

A sintered body containing the piezoelectric composition represented bythe above Chemical Formula (1) is obtained by sintering the formed bodyafter the binder removal treatment. The sintering temperature may beabout 900 to 1100° C., and the sintering time may be adjusted to around2 to 20 hours. The binder removal treatment and the sintering of theformed body may be performed successively, or may be performedseparately.

The surface of the obtained sintered body is polished if needed, and thesintered body is processed such that the sintered body is cut to formthe desired shape of a piezoelectric body 2. A pair of electrodes 4 aand 4 b is formed on both surfaces of the processed sintered body. Theelectrodes 4 a and 4 b may be formed by applying and baking an electrodepaste, or may be formed by vapor deposition or sputtering filmformation.

A piezoelectric device 10 according to this embodiment can be obtainedby impressing a polarization electric field on the sintered body onwhich the electrodes are formed and performing a poling process. Theconditions of the poling process may be properly determined depending onthe constitution of the piezoelectric composition that the sintered bodycontains. The conditions of the poling process may be, for example, asfollows. The sintered body may be immersed into silicone oil at 25 to200° C. The time to impress a polarization electric field may be 5 to 60minutes. The magnitude of a polarization electric field may be 1.2 ormore times greater than that of the coercive electric field of thesintered body.

The uses of a piezoelectric composition according to this embodiment arevarious. The piezoelectric composition may be applied, for example, to aradiator, a resonator, an actuator, a motor or a sensor. The specificuse of the piezoelectric composition may be, for example, a SAW filter,a BAW filter, a piezoelectric microphone, a head assembly, a hard diskdrive, a printer head, an ink jet printer device, an ultrasonic washingmachine, an ultrasonic motor, an atomizer oscillator, a fish finder, ashocking sensor, an ultrasonic diagnostic device, a waste toner sensor,a gyro sensor, a buzzer, a transformer, or a lighter.

As mentioned above, preferable embodiments of the present invention weredescribed, but the present invention is not necessarily limited to theembodiments mentioned above. Various modifications of the presentinvention are possible as long as the modifications do not deviate fromaims of the present invention, and these modification examples areincluded in the present invention.

EXAMPLES

The present invention will be described in detail by using Examples andComparative Example hereinafter. However, the present invention is notlimited to the following Examples at all.

Example 1

A piezoelectric device was manufactured by a method illustrated below.As raw materials of the piezoelectric composition, raw material powdersof bismuth oxide (Bi₂O₃), iron oxide (Fe₂O₃), barium carbonate (BaCO₃),strontium carbonate (SrCO₃), and titanium oxide (TiO₂) were provided.These raw material powders were weighed and blended, and the mixed rawmaterial was prepared such that a sintered body after the finalsintering became a piezoelectric composition represented by thefollowing Chemical Formula (3).

x[Bi_(m)FeO₃]-y[Ba_(m)TiO₃]-z[Sr_(m)TiO₃]  (3)

In the above Chemical Formula (3), x=0.58, y=0.4, z=0.02, and m=1.00.

Then, the prepared mixed raw material and pure water were mixed with Zrballs by a ball mill for 10 hours to obtain slurry. This slurry wasfully dried, thereafter press-formed and calcined at 800° C. to obtain acalcined body. Next, the calcined body was pulverized by the ball milland thereafter dried, a proper amount of PVA (polyvinyl alcohol) wasadded thereto as a binder, and the mixture was granulated. Around 3 g ofthe obtained granulated powder was placed into a metal mold of 20 mm inlength by 20 mm in width and formed under a load of 3.2 MPa by using auniaxial press molding machine.

The molded sample was heat-treated to remove the binder therefrom andthereafter finally sintered at 950 to 1100° C. for 4 hours to obtain asintered body.

The obtained sintered body was flattened to a thickness of 0.5 mm with adouble-side lapping machine and thereafter cut to a size of 16 mm inlength by 16 mm in width with a dicing saw. Subsequently, electrodes 4 aand 4 b were formed on both surfaces of the sintered body by using avacuum evaporator. The electrodes 4 a and 4 b were composed of 1.5-μm Aglayers. The sizes of the electrodes were 15 mm by 15 mm.

Thereafter, a poling process in which 1.5 to 2 times as strong anelectric field as the coercive electric field was impressed on thesintered body on which the electrodes were formed in a silicone oil bathat a temperature of 120° C. for 15 minutes was performed to obtain apiezoelectric device 10 having the same configuration as FIG. 1.

[Analysis of Constitution]

The constitution of the sintered body in Example 1 was analyzed by anX-ray fluorescence analysis (XRF method). Consequently, it has beenconfirmed that the constitution of the sintered body in Example 1 isrepresented by the above Chemical Formula (3).

[Measurement of d₃₃]

The d₃₃ (unit: pC/N) of the sintered body (piezoelectric composition) inExample 1 before the forming of temporary electrodes was measured byusing a d₃₃ meter. The above d₃₃ meter measures d₃₃ by the Berlincourtmethod based on JIS R 1696. In the Berlincourt method, the d₃₃ ismeasured by utilizing a piezoelectric positive effect at the time whenvibration is given to a piezoelectric composition. Therefore, in theBerlincourt method, there is no influence of electrostrictiondifferently from a measuring method in which a piezoelectric reverseeffect at the time when an electric field is impressed on apiezoelectric composition is utilized, and the original d₃₃ of apiezoelectric composition is obtained. The d₃₃ in Example 1 isillustrated in Table 1. The d₃₃ is good when it is 150 pC/N or more.

[Measurement of Specific Resistance]

Direct-current voltage was impressed between the electrode 4 a and theelectrode 4 b in the piezoelectric device in Example 1, and the specificresistance (unit: Ω·cm) between the electrode 4 a and the electrode 4 bwas measured. The direct-current voltage was 40 V. The specificresistance in Example 1 is illustrated in Table 1. The specificresistance is good when it is 1.0×10⁹ Ω·cm or more.

Examples 2 to 11 and Comparative Examples 1 to 6

Respective piezoelectric devices in Examples 2 to 11 and ComparativeExamples 1 to 6 were individually manufactured by the same method as inExample 1 except that the ratios of powder raw materials blended werealtered such that x, y, z, and m in the above Chemical Formula (3)became values illustrated in the following Table 1 or 2 when mixed rawmaterials were prepared. The respective constitutions of the sinteredbodies in Examples 2 to 11 and Comparative Examples 1 to 6 were analyzedby the same method as in Example 1. Consequently, it has been confirmedthat the respective constitutions of the sintered bodies in Examples 2to 11 and Comparative Examples 1 to 6 are the constitutions illustratedin Table 1 or 2. The d₃₃s of Examples 2 to 11 and Comparative Examples 1to 6 were measured individually by the same method as in Example 1. Thed₃₃s of Examples 2 to 11 and Comparative Examples 1 to 6 are illustratedin Table 1 or 2. The specific resistances in Examples 2 to 11 and theComparative Examples 1 to 6 were measured individually by the samemethod as in Example 1. The specific resistances in Examples 2 to 11 andComparative Examples 1 to 6 are illustrated in Table 1 or 2. InComparative Example 5, since the sintered body had broken when a polingprocess was performed, the d₃₃ and the specific resistance were notmeasured.

[Table 1]

TABLE 1 Specific resistance BiFeO₃ BaTiO₃ SrTiO₃ A/B d₃₃ (pC/N) (Ω · cm)x y z m — — Example 1 0.58 0.4 0.02 1.00 201 2.5 × 10¹² Example 2 0.80.18 0.02 1.00 211 8.9 × 10¹¹ Example 3 0.78 0.02 0.2 1.00 168 7.6 ×10¹¹ Example 4 0.5 0.3 0.2 1.00 152 3.1 × 10¹² Example 5 0.65 0.33 0.021.00 232 3.1 × 10¹² Example 6 0.75 0.23 0.02 1.00 222 1.0 × 10¹² Example7 0.7 0.15 0.15 1.00 190 7.6 × 10¹¹ Example 8 0.55 0.3 0.15 1.00 216 5.5× 10¹¹ Example 9 0.65 0.25 0.1 1.00 219 2.2 × 10¹² Comparative 0.6 0.4 01.00 195 2.1 × 10⁸  Example 1 Comparative 0.8 0.2 0 1.00 210 1.2 × 10⁸ Example 2 Comparative 0.8 0 0.2 1.00 135 9.3 × 10¹⁰ Example 3Comparative 0.48 0.32 0.2 1.00 121 1.2 × 10¹² Example 4

TABLE 2 Specific resistance BiFeO₃ BaTiO₃ SrTiO₃ A/B d₃₃ (pC/N) (Ω · cm)x y z m — — Example 9 0.65 0.25 0.1 1.00 219 2.2 × 10¹² Example 10 0.690.4 0.1 0.96 175 9.8 × 10⁹  Example 11 0.69 0.4 0.1 1.04 170 4.4 × 10¹¹Comparative 0.5 0.3 0.2 0.94 — — Example 5 Comparative 0.65 0.33 0.021.06 145 6.8 × 10¹¹ Example 6

FIG. 2 is a schematic view illustrating the constitutions of thepiezoelectric compositions in Examples and Comparative Examples. BFmeans BiFeO₃ in FIG. 2. BT means BaTiO₃. ST means SrTiO₃. Theconstitutions of the piezoelectric compositions are illustrated bycoordinates (x, y, z) in FIG. 2. A number appended to a round mark isthe number of Example, and a number appended to a triangle mark is thenumber of Comparative Example. That is, the coordinates of a round markindicate the constitution of a piezoelectric composition in Example, andthe coordinates of a triangle mark indicate the constitution of apiezoelectric composition in Comparative Example. The coordinates (x, y,z) of Example 1 are (0.58, 0.4, 0.02). The coordinates (x, y, z) ofExample 2 are (0.8, 0.18, 0.02). The coordinates (x, y, z) of Example 3are (0.78, 0.02, 0.2). The coordinates (x, y, z) of Example 4 are (0.5,0.3, 0.2). A range surrounded by dotted lines connecting the coordinatesof Examples 1 to 4 is defined as a range A. The range A is in the rangeof the constitutions of the piezoelectric compositions according to thepresent invention. The coordinates (x, y, z) of Example 5 are (0.65,0.33, 0.02). The coordinates (x, y, z) of Example 6 are (0.75, 0.23,0.02). The coordinates (x, y, z) of Example 7 are (0.7, 0.15, 0.15). Thecoordinates (x, y, z) of Example 8 are (0.55, 0.3, 0.15). A rangesurrounded by dashed lines connecting the coordinates of Examples 5 to 8is defined as a range B. The piezoelectric compositions belonging to therange B are superior to the piezoelectric compositions belonging to theabove range A in the point that at least either of the d₃₃ and thespecific resistance is likely to become large.

From the above experimental results, it has been confirmed that all theExamples are in the range of the piezoelectric composition representedby the above Chemical Formula (1). Meanwhile, it has been confirmed thatall the Comparative Examples are out of the range of the piezoelectriccomposition represented by the above Chemical Formula (1).

As illustrated in Tables 1 and 2, the d₃₃s of all the Examples were 150pC/N or more, and the specific resistances of all the Examples were1.0×10⁹ Ω·cm or more. Meanwhile, there was no Comparative Examples thed₃₃s of which were 150 pC/N or more and the specific resistances ofwhich were 1.0×10⁹ Ω·cm or more. It has been confirmed that apiezoelectric composition the d₃₃ of which is large and the specificresistance of which is large and a piezoelectric device using thepiezoelectric composition are provided according to the presentinvention.

REFERENCE SIGNS LIST

2: piezoelectric body; 4 a, 4 b: electrode; 10: piezoelectric device.

What is claimed is:
 1. A piezoelectric composition represented by afollowing Chemical Formula (1):x[Bi_(m)FeO₃]-y[Ba_(m)TiO₃]-z[Sr_(m)TiO₃]  (1) wherein 0.5≤x≤0.8,0.02≤y≤0.4, 0.02≤z≤0.2, x+y+z=1, and 0.96≤m≤1.04.
 2. A piezoelectricdevice comprising: the piezoelectric composition according to claim 1.